The present invention relates generally to lighting, and more specifically to an arc discharge lamp, such as a ceramic metal halide lamp. This invention relates particularly to novel conducting materials and a means for applying such conducting members to high pressure arc discharge lamp tubes, e.g. sodium arc tubes (HPS).
Discharge lamps produce light by ionizing a fill such as a mixture of metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill are sealed within a translucent or transparent discharge chamber which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill, also known as a xe2x80x9cdosexe2x80x9d emits a desired spectral energy distribution in response to being excited by the electric arc. However, arc discharge lamps, particularly those of a high pressure variety, are often difficult to start.
Previous HPS starting aids have been designed in the form of a wire or coiled ignition filament. The starting aid is positioned in contact with the outer surface of the arc tube and is connected to one electrical power lead of the lamp. When an arc is formed and the lamp begins to warm up, power is either removed form the starting aid, or the starting aid is moved away from the arc tube, so as to prevent electric field accelerated sodium diffusion through the arc tube wall. Such sodium diffusion would adversely affect the lamp life. One drawback to this method for applying an external conducting member to HPS arc tubes is the cost and complexity of designing lamps with movable starting aids. Furthermore, in the absence of direct attachment to the lamp, the starting aid may sag away from the arc tube due to the high temperature of operation. Additionally, these switches are typically attached to the lamp frame, resulting in heating by radiation, rather than by conduction. This results in variation of lamp performance depending on the wattage of different lamps.
Alternative metallic starting aids for discharge lamps are composed of tungsten metal to reduce the required electrical breakdown voltage for starting. Unfortunately, they may not survive an air firing step used for cleaning ceramic arc tubes. Moreover, the tungsten oxidizes and becomes useless as a starting aid. A similar oxidation problem prevents application of metallic starting aids on arc tubes intended for open air operation, such as ceramic or quartz mercury arc tubes for light projectors. Another problem with existing metallic starting aids is that tungsten is relatively expensive. However, lower cost metals have higher vapor pressure, which in the vacuum established within the outer envelope of HPS lamps, would evaporate to produce bulb darkening and lumen reduction.
It would therefore be desirable to find a novel conducting member, and a means for applying such a conducting member to arc tubes, which would overcome the above mentioned problems.
The present invention relates to an arc discharge lamp comprising an arc tube including a starting aid. The starting aid comprises at least one conducting compound applied to the surface of the arc tube. The conducting compound is comprised of metal oxides, nitrides, carbides, and silicides. Particularly preferred materials include titanium nitride, silicon carbide, tungsten carbide, molybdenum silicides (Mo5Si3 and MoSi2), silicon nitrides, and molybdenum carbides.
In another embodiment of the present invention, a method for forming an arc discharge lamp is provided. This method comprises applying as particles, or alternatively sputter coating, the metal oxides, nitrides, carbides, and silicides onto the surface of an arc tube.
In another embodiment of the present invention, a high pressure sodium lamp is provided. The high pressure sodium lamp includes an outer bulb; first and second discharge devices within said outer bulb connected electrically in series, each discharge device including a discharge vessel enclosing a discharge space and an ionizable filling; first and second discharge electrode assemblies within said discharge space each including an electrode portion on which a discharge arc terminates during normal lamp operation and a current conductor portion extending to the exterior of said discharge vessel; means for electrically connecting said first electrode assembly of each discharge device to a source of electric potential outside of said lamp envelope; and a starting aid including a coating on the surface of the arc tube. The coating is comprised of metal oxide, nitride, carbide, or silicide.